1. Field of the Invention
The present invention relates to a Micro Electro-Mechanical System (MEMS) gyroscope for sensing angular velocity and angular acceleration generated due to the rotation of a movable body, and more particularly to a tuning fork vibratory MEMS gyroscope, withstanding a noise at a peripheral area, which is driven at external portions of internal frames, is sensed at internal portions of the internal frames, and comprises elastic members having a wine glass shape, thereby being more stably operated.
2. Description of the Related Art
Generally, gyroscopes are used as sensors for sensing angular velocity or angular acceleration generated due to the rotation of a movable body. The gyroscopes are divided into mechanical gyroscopes and vibratory gyroscopes according to types of force applied thereto, and divided into ceramic gyroscopes and MEMS gyroscopes, using a semiconductor manufacturing process, according to types of manufacturing process. Further, the vibratory gyroscopes are divided into lateral gyroscopes and vertical gyroscopes according to directions of the applied force. Here, the lateral gyroscope uses Coriolis' force applied in a direction horizontal with the horizontal plane of velocity, and the vertical gyroscope uses Coriolis' force applied in a direction vertical to the horizontal plane of velocity.
The above Coriolis' force, used by the vibratory gyroscopes, is obtained by the equation “Fc=2 mΩ·V”. Here, “m” represents the weight of a movable body, “Ω” represents angular velocity, and “V” represents velocity. The direction of the Coriolis' force (Fc) is determined by an axis of the velocity (V) and a rotary axis of the angular velocity (Ω). The vibratory gyroscopes are applied to apparatuses for sensing vibration (for example, hand trembling) and compensating for the vibration.
The vibratory gyroscope comprises stationary structures and vibratory structures. The stationary structures include driving structures and sensing structures. The driving structures serve to resonate the vibratory structures by means of magnetic oscillation for forming sensing conditions in a driving mode, and the sensing structures serve to resonate the vibratory structures by means of the Coriolis' force (Fc) applied in the direction perpendicular to the angular velocity or the angular acceleration corresponding to the movement of a movable body (for example, hand-held trembling of a digital camera). The direction of resonance of the vibratory structures in the driving mode and the direction of resonance of the vibratory structures in the sensing mode are perpendicular to each other. Here, the size of a capacitor according to the degree of the Coriolis' force (Fc), i.e., the degree of the trembling of the movable body, is measured.
In the sensing mode of the gyroscope, detection of the voltage of the gyroscope is achieved by a method for measuring capacitance corresponding to the Coriolis' force and then converting the capacitance into voltage, and a rebalance method for measuring voltage required to suppress movement caused by the Coriolis' force.
In order to improve a sensing capacity of the above vibratory gyroscope, the movement of the gyroscope must be large in the driving mode and the sensitivity of the gyroscope must be excellent.
The vibratory gyroscope is applied to an apparatus for preventing a camcorder from trembling, a roll-over airbag for vehicles, a toy pilotless airplane, and a head mount display (HMD).
In the case that the vibratory gyroscope, serving as a sensor for measuring angular velocity in a designated direction, is applied to the above-described apparatuses, the vibratory gyroscope needs to be insensitive to angular velocities or movements in other directions, not to be measured. The sensitivity to the above movements in other directions, not to be measured, is defined as “cross talk” or “cross sensitivity”. Sensors for measuring physical quantities must minimize their cross sensitivity, and limit the cross sensitivity to a designated value defined in their specifications.
U.S. Pat. No. 5,747,690A discloses a conventional micro gyroscope. The above-disclosed micro gyroscope is driven using combs aligned in the horizontal direction of the X-axis, and the vibration of the micro gyroscope generated by the Coriolis' force in the direction of the vertical direction of the Y-axis is sensed using sensing electrodes. That is, in the case that a suspended weight is vibrated in the direction of the X-axis by applying AC current to the combs formed on both surfaces of the suspended weight, when an angular velocity in the direction of the Z-axis is applied, the weight is vibrated at a vibrating frequency by the Coriolis' force in the direction of the Y-axis. Here, the vibrating range of the weight is in direct proportion to the applied angular velocity, and the vibrating frequency of the vibration of the weight in the direction of the Y-axis is detected using the sensing electrodes. Thereby, an angular velocity signal is obtained.
In the driving mode of the above conventional horizontal micro gyroscope, the weight is initially vibrated in the direction of the X-axis, and is then vibrated in the direction of the Y-axis due to the Coriolis' force generated by the external angular velocity applied thereto, thereby sensing an angular velocity. When the angular velocity is detected by the above method, the vibration is transmitted in the directions of the X-axis and Y-axis, particularly in the direction of the sensitive Y-axis, thereby being directly outputted. There is a component, which is weak to a vibration close to a characteristic vibration frequency out of external vibrations, and is modulated by a signal of the characteristic vibration frequency (resonant frequency) to generate an angular velocity signal, while vibrations at other frequency bands are decreased using an electrical filter. The component cannot be electrically offset, thereby deteriorating the sensing capacity of the gyroscope.
U.S. Pat. No. 5,349,855A discloses a conventional tuning fork micro gyroscope. The above-disclosed tuning fork micro gyroscope uses a tuning fork vibrating mode. In the tuning fork mode, the sensing is performed in the direction vertical to the ground surface. This micro gyroscope comprises a structure, which performs a driving mode in the horizontal direction and a sensing mode in the vertical direction. When frequencies in the above two directions are the same, the above structure generates the highest output. Accordingly, after the manufacturing process of the gyroscope, a tuning process, in which the two resonant frequencies in the horizontal and vertical directions are set to similar values or the same value through an electrical step, is performed.
When the two resonant frequencies in the horizontal and vertical directions are set to the same value, since the directions of sensing and driving modes are respectively horizontal and vertical, elastic bodies, such as springs, vibrated in the horizontal and vertical directions must have the same height and thickness, thereby causing a difficulty in performing the frequency-tuning process. Since the frequency in the horizontal direction is sensitive to an etching process of the structure and the frequency in the vertical direction is determined by a depositing, plating, or polishing process determining the thickness of the structure, the two processes must be carefully controlled. However, it is difficult to substantially control the processes. Further, since a sensing electrode is formed biased to one side, a high angular velocity is applied to the sensing electrode to increase the degree of the vibration of the weight in the vertical direction, thereby causing nonlinearity in measurement.
As described above, in the conventional gyroscopes, external vibration or noise is transmitted to structures of the gyroscope, thereby increasing signals other than the signal regarding the angular velocity. The above generation of abnormal signals has a negative influence on functions of products employing the gyroscopes.
In order to solve the above conventional problems, a horizontal and tuning fork vibratory gyroscope, as shown in FIG. 1, is proposed.
FIG. 1 is a plan view of a conventional horizontal and tuning fork vibratory gyroscope.
With reference to FIG. 1, the conventional horizontal and tuning fork vibratory gyroscope comprises a substrate 105, a fixed portion 110, an external elastic member 120, an external frame portion 130, a sensing electrode portion 140, an internal elastic member 150, an internal mass portion 160 having a pair of first and second internal weights respectively including driving combs, and a driving electrode portion 170 including a comb driver.
Here, when a driving signal is supplied to the driving electrode portion 170, electrostatic force is generated between the driving electrode portion 170 and the internal mass portion 160, thereby driving the first and second internal weights, which face each other in the Y-axis direction, of the internal mass portion 160 to a horizontal tuning fork mode such that that the first and second internal weights reciprocate so as to be close to and distant from each other.
When the gyroscope of FIG. 1 generates angular velocity having a rotary axis vertical to the X and Y axes, a pair of the internal weights are vibrated by Coriolis' forces in reverse directions along the X axis, and the vibration of the internal weights is transmitted to the external frame portion 130 through the internal electric member 150, thereby causing the external frame portion 130 to be vibrated.
In this case, the sensing electrode portion 140 detects capacitance corresponding to a variation in an interval between the external frame portion 130 and the sensing electrode portion 140, thereby sensing the degree of external force or its self vibration.
Japanese Patent Laid-open Publication No. 2004-205492 discloses the detailed description of the above horizontal and tuning fork vibratory gyroscope.
The above conventional turning fork vibratory gyroscope is operated in a horizontal mode, in which a resonance direction of a sensing mode and a resonance direction of a driving mode are on the same plane, and is insensible to a variation in the thickness in the vertical direction determined in a manufacturing process, thereby being advantageous in that sensing characteristics of the gyroscope are improved.
However, the conventional horizontal and tuning fork vibratory gyroscope as shown in FIG. 1 has several problems, as follows.
FIG. 2 is a schematic view illustrating a “π”-shaped spring in the tuning fork mode of the gyroscope of FIG. 1.
With reference to FIGS. 1 and 2, the external frame portion 130 and the first and second internal weights of the internal mass portion 160 of the conventional horizontal and tuning fork vibratory gyroscope are connected by “π”-shaped springs. When the gyroscope is operated in the tuning fork mode, a body portion of the “π”-shaped spring, as shown in FIG. 2, is unstably deformed, thereby causing unstable tuning fork operation of the gyroscope. More severely, the unstable deformation of the body portion of the “π”-shaped spring causes abnormal oscillation of the gyroscope.
Further, since the conventional horizontal and tuning fork vibratory gyroscope, as shown in FIG. 1, comprises a plurality of small elements, which must be precisely finished, it is difficult to manufacture the conventional horizontal and tuning fork vibratory gyroscope and manufacturing the conventional horizontal and tuning fork vibratory gyroscope is costly. Particularly, since the above gyroscope comprises plural mechanical elements, it is difficult to apply gyroscope to an integrated circuit-type product.